High-Performance Lignin-Containing Polyurethane Elastomers with

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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High-Performance Lignin-Containing Polyurethane Elastomers with Dynamic Covalent Polymer Networks Weifeng Liu,*,†,§ Chang Fang,†,§ Shengyu Wang,† Jinhao Huang,† and Xueqing Qiu*,†,‡ †

School of Chemistry and Chemical Engineering, Guangdong Engineering Research Center for Green Fine Chemicals, and ‡State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan Road 381, Guangzhou, Guangdong 510640, China

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S Supporting Information *

ABSTRACT: High-performance lignin-containing polyurethane elastomers (LPUes) were successfully synthesized via partially substituting petroleum-derived polyols with depolymerized enzymatic hydrolysis lignin (DEL). The influences of DEL on the structure, thermostability, mechanical performance, and thermal reprocessability of LPUes were systematically studied. The tensile strength and toughness of PUes were significantly enhanced after the introduction of DEL, with the maximum tensile strength reaching up to 60.7 MPa and the toughness up to 263.6 MJ/m3. The enhancement for the strength and toughness was attributed to the dual cross-linking network structure and the interfacial hydrogen bonds between lignin and the PU matrix, which were demonstrated to facilitate the orientation of chain segments and lead to straininduced crystallization and self-reinforcement. LPUes also exhibited much better elasticity than the control sample without lignin and could maintain excellent mechanical performance after being hot reprocessed.

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INTRODUCTION

Lignin, the second largest biomass resource from plants, has attracted increasing attention because of its wide origin and low cost. Despite its abundance, lignin is still of little utilization nowadays and is mainly treated as the waste residue of pulping and biorefinery industries, which results in huge resource waste and environmental pollution. In the past decade, some research studies have focused on using lignin to synthesize PU materials based on the abundant hydroxyl groups in lignin molecules. However, because of its complex structure and low reactivity, lignin has been mainly applied in the field of rigid PU foams.9 For example, Wang et al.8 modified lignin by oxidation and synthesized lignin-derived polycarboxylic acids, which was then applied in rigid PU foams. Zhang et al.10 functionalized kraft lignin with polyisocyanate and then applied the modified lignin in the synthesis of rigid PU foams. However, the complex modification process increased the production cost, and the rigid PU foams are thermoset and nonrecyclable and of short life cycle. Research studies focusing on the application of lignin in PUes have also been conducted by scholars. For example, Zhang et al.11 modified lignin with butyric anhydride and octadecyl isocyanate, which improved its compatibility with the PU matrix; however, the PU composite with modified lignin exhibited poor performance with a strength descent. Li

Polyurethane (PU), a block copolymer made of polyisocyanates and macropolyols, has been ubiquitously applied in many fields such as foams, elastomers, coatings, and adhesives because of its unlimited performance possibilities. The magic performance of PU originated from its various commercially available raw materials, highly flexible formulation compositions, and widely adjustable molecular structures.1,2 However, most of the commercial PUs come from nonrenewable petroleum resources. The rising awareness of environmental protection has driven the urgent need for sustainable development. Searching alternative biomass resources for the synthesis of PUs has become a hotspot for pursuing green PU materials.3,4 For example, Zhang et al.5 used hydrophobic biobased polyfarnesene diol to synthesize PU elastomers (PUes) with excellent hydrolysis resistance. Besides, Zhang6 also prepared PUes by completely substituting petroleumbased polyols with renewable castor oil-based polyols. Alagi7 prepared polyols with controlled hydroxyl functionalities from soybean oil and used them to synthesize thermoplastic PU (TPU) with good shape memory properties. However, the limited output and relatively high cost of vegetable oil make the research studies on producing biobased chemicals from crops controversial because they may lead to the exacerbation of the global food crisis.8 Therefore, searching other nonfood and cheap biomass resources for the synthesis of PU becomes particularly important. © XXXX American Chemical Society

Received: July 6, 2019 Revised: August 3, 2019

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DOI: 10.1021/acs.macromol.9b01413 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules et al.12 prepared lignin-containing PUes (LPUes) by reacting the PU prepolymer with the lignin of different molecular weights, which was proved to have strengthening effects on the PUes. Saito et al.13 synthesized PUes containing up to 65−75 wt % lignin, but the comprehensive performance was poor. It is noticeable, however, that few research studies have been conducted on applying lignin in thermally reprocessable PUes. This is due to the fact that lignin plays the role of not only polyol components but also chemical cross-linking agents because of its abundant hydroxyl groups. Therefore, PUes containing lignin are almost chemically cross-linked thermoset elastomers. Recently, to overcome the drawback of chemically crosslinked thermosets, dynamic covalent bonds that are reversible at certain conditions have been introduced into polymers to construct dynamic cross-linking networks. Given appropriate stimulation, dynamic covalent bonds could undergo reversible exchange reactions and the chemical cross-linking structures were temporarily broken, endowing dynamic chemical crosslinked thermosets with characteristics of thermal reprocessability, good fluidity at high temperature, and even self-healing property.14,15 There are various dynamic covalent bonds including Diels−Alder reaction,16−18 transesterification reaction,19,20 alkene metathesis reaction,21,22 disulfide exchange reaction,23−26 transcarbamoylation,27 dynamic urea bond,28 and so on. In PUs, transcarbamoylation reactions could happen at certain temperature when catalysts exist.27 Specially, the phenolic urethane bonds could undergo transcarbamoylation reaction more easily. Herein, by taking advantage of the phenolic urethane bonds, we made high-performance LPUes with thermal reprocessing ability using partly depolymerized enzymatic hydrolysis lignin (DEL) as a partial substitute for petroleum-derived polyols. Enzymatic hydrolysis lignin (EL) was first depolymerized with the base catalyst to reduce the molecular weight and increase the hydroxyl group content, which was beneficial to increase the reactivity of lignin with isocyanate. Systematic study was conducted on the effects of DEL on the structure, mechanical performance, thermostability, and thermal reprocessing ability of the LPUes.

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After the reaction, the product was poured into a polytetrafluoroethylene mold and kept at room temperature overnight, and then it was heated to 70 °C for 12 h to evaporate the solvent. Finally, the sample was vacuum-dried at 70 °C for 12 h. The dried sample (1.5 g) was put into the mold (50 × 50 × 0.6 mm3), preheated for 5 min, and hot pressed at 160 °C, 5 MPa for 5 min to get the flat sheet for characterizations. The feeding formula is listed in Table 1. The

Table 1. Formulation of PUes with Different Amounts of DEL samples

PTMEG (g)

DEL (g)

HDI (g)

DBTDL (g)

control sample 5% DEL 10% DEL 10% EL 20% DEL 40% DEL 60% DEL

5.00 4.75 4.50 4.50 4.00 3.00 2.00

0.67 (APD) 0.25 0.50 0.50 1.00 2.00 3.00

0.991 0.621 0.738 0.717 0.971 1.437 1.903

0.03 0.03 0.03 0.03 0.03 0.03 0.03

isocyanate index (the molar ratio of −NCO groups in HDI to −OH groups in PTMEG2000 and DEL) was kept at 1.2. The sample of the synthesized polyurethane elastomers was labeled according to the weight substitution amount of DEL for PTMEG2000. For example, the sample named 5% DEL means 5 wt % of PTMEG2000 was substituted by DEL. Particularly, APD other than DEL was used to prepare the control sample. Characterizations. All samples were vacuum-dried at 50 °C for 12 h before characterizations. Gel permeation chromatography analysis was conducted to characterize the relative molecular weight distribution of EL and DEL. Lignin solution of 1 mg/mL in chromatographical grade tetrahydrofuran (THF) was filtrated through a 0.45 μm filter membrane. THF was employed as the solvent at a flow rate of 1.0 mL/min. 1 H NMR analysis was conducted on a Bruker NMR spectrometer (AVANCE III HD 400 MHz) in DMSO-d6 to characterize the structural variation of lignin before and after depolymerization. 31P NMR spectra were recorded by a Bruker AVANCE III HD 600 MHz spectrometer to quantitatively analyze the content of functional groups. Anhydrous mixture of pyridine/deuterium chloroform with a volume ratio of 1.6:1 was prepared before use, where 4A molecular sieves were added. Chromium acetylacetonate solution (4.9 mg/mL) was prepared as a relaxation agent, and 11.2 mg/mL of cyclohexanol solution was prepared as an internal standard. Lignin (30 mg) was dissolved in 0.5 mL of pyridine/deuterium solution, and then, 100 μL of cyclohexanol solution, 100 μL of chromium acetylacetonate solution, and 50 μL of 4,4-trimethylenedipyridine were added to the solution. The mixed solution was injected into the nuclear magnetic pipe for test.29−31 The test was conducted at 104 scan times, 45° pulse angle, and pulse delay time of 25 s. A Fourier transform infrared (FT-IR) spectrometer (Bruker, VERTEX 70) was used to characterize the structure of EL, DEL, and LPUe (in attenuated total reflection pattern). The test was conducted with 64 scan times in a scan range of 4000−400 cm−1. The FT-IR spectrometer (Nicolet NEXUS 670) was also used for the structural characterization of 20% DEL at different temperatures. The mechanical properties of LPUe were tested at room temperature with a CMT electronic universal testing machine (MTS, China). The hot-pressed sample (50 × 50 × 0.6 mm) was first cut into a dumbbell-shaped spline with a length of 50 mm, a narrow section width of 4 mm, and a thickness of 0.6 mm. The stretching rate was set at 200 mm/min. Young’s modulus was calculated at the strain of 5%, and the toughness was calculated as the integral area under the stress−strain curve according to formula FS2. The elastic recovery represented the percentage of the recovery deformation, which was calculated via formula FS3. The hysteresis property of PUes was tested according to two programs with a CMT electronic universal testing machine (MTS, China). Hysteresis performance at fixed strain: the LPUe samples

EXPERIMENTAL SECTION

Materials. EL was purchased from Longli Biotechnology Co., Ltd. (Shandong, China). All reagents were provided in chemical grade. Polytetramethylene ether glycol (PTMEG2000), dibutyltin dilaurate (DBTDL), sodium hydroxide (NaOH), hexamethylene diisocyanate (HDI), and N,N-dimethylacetamide (DMAC) were purchased from Aladdin. Bis(4-hydroxyphenyl)disulfide (APD) was provided by Energy Chemical. Depolymerization of EL. EL (500 g) was added into 3000 g of aqueous alkali solution (2 mol/L NaOH). The reaction lasted for 10 h at 155 °C and 0.6 MPa. The product was then poured out after cooling to room temperature and filtrated to remove insoluble impurities. Then, pH was adjusted to 3.0 to precipitate lignin from the alkali liquor. Lignin was filtrated and constantly rinsed with deionized water until the pH of the filtrate reached 5.0. Finally, the lignin was vacuum-dried at 50 °C and ground to get the solid powder of DEL. Preparation of LPUes. LPUe was prepared according to the following procedures. PTMEG2000 was first added into a flask, heated to 120 °C, and vacuum-dried for 2 h to remove the moisture. Then, the temperature was decreased to 70 °C, and 10 mL of DMAC was added to the flask. DBTDL (0.03 g) and HDI were added sequentially. The reaction lasted for 2 h at 70 °C to get the PU prepolymer. After the prepolymerization, DEL in 10 mL of DMAC solution was added into the flask, and the reaction continued for 4 h. B


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Macromolecules were fixed at the strain of 600%, while the control sample was fixed at the strain of 550%. All samples were first stretched to the fixed strain at a rate of 50 mm/min, and then, they were unloaded to the initial state at the same rate. The process was repeated 10 times. After the last cycle, the sample was heated at 40 °C for 1 min to recover the residual strain, after which the sample was stretched and unloaded one more time. The dissipative energy was calculated as the area surrounded by the load−unload curve. Hysteresis performance at various strains: the hysteresis property of LPUe samples was tested at different strains including 50, 100, 200, 300, 400, 600, 800, 1000, and 1200%. The sample was stretched and unloaded at the rate of 50 mm/min. X-ray diffraction (XRD) test for PUes was conducted with an X-ray diffractometer (PANalytical, X’pert3 Powder) using copper radiation, and the incident wavelength was 0.15418 nm. The scanning step was 0.013°, and the scanning speed was 15 s/step. Small-angle X-ray scattering (SAXS) experiment was conducted on a Xenocs Xeuss 2.0 instrument (MetalJet-D2, Excillum). The wavelength was set as 1.34 Å. The distance from samples to detector (Pilatus3 R 1M, Dectris) was 2506.75 mm. The two-dimensional (2D) SAXS figures were transformed to one-dimensional (1D) curves via Fit2D software. The stress relaxation behavior of LPUes was studied by a rotational rheometer (HAAKE, MARS III) with a parallel plate clamp. The sample was cut into a disk of 25 mm in diameter and 0.6 mm in thickness. Before testing, the sample was put on the parallel plate at the preset temperature to stabilize for 5 min, after which a 2 N force was put to ensure the complete touch between the sample and parallel plate. Then, 5% strain was set for testing, and the measuring time ranged from 0.01 to 1000 s. The thermal degradation behavior of LPUe samples was measured by a thermogravimetric analyzer (NETZSCH, TG 209 F1 Libra) at nitrogen atmosphere. The samples were heated from room temperature to 600 °C at the rate of 10 °C/min. Differential scanning calorimetry (DSC) analysis for PUes was conducted at nitrogen atmosphere on TA DSC2500. The sample was first heated from room temperature to 180 °C at the rate of 50 °C/min and kept for 3 min to eliminate the effect of heat history, after which the sample was cooled to −90 °C at the rate of 10 °C/min and kept for 5 min, and then the sample was reheated from −90 °C to 180 °C at the rate of 10 °C/ min. The crystallization temperature (Tc) was recorded from the cooling curve; the melting temperature (Tm) and the melting enthalpy (ΔHm) were acquired from the reheating curve.

increase the reactivity of DEL and construct more hydrogen bonds in the PU matrix.12 LPUes were synthesized by partially replacing PTMEG2000 with DEL. The reaction mechanism is illustrated in Figure 1.

Figure 1. Synthesis of LPUes.

The FT-IR spectra of LPUe with different amounts of DEL were similar, as shown in Figure 2a. The vibration of CO in urethane groups was observed at the absorption peaks of 1720 cm−1 (disordered hydrogen-bonded) and 1701 cm−1 (ordered hydrogen-bonded). The absorption peaks at around 1681 and 1620 cm−1 were attributed to the vibration of CO in free urea bonds and hydrogen-bonded urea bonds, respectively.34 Apparent peaks of urea CO bonds were observed in the control sample, indicating that there were amounts of carbamido groups in the control sample. This was due to the reaction between the excess isocyanate and urethane bonds in the presence of a catalyst with the isocyanate index of 1.2.35 After partially substituting polyols with DEL, the peaks at 1620 and 1681 cm−1 obviously weakened, indicating that the content of urea bonds significantly reduced, whereas the peaks at 1720 and 1701 cm−1 strengthened, indicating the increase in the content of urethane bonds. Besides, with the increasing content of DEL, the peak intensity at 1720 cm−1 weakened at 10% DEL and then strengthened, whereas the peak intensity at 1701 cm−1 strengthened at 10% DEL and then weakened, which indicated that introduction of appropriate amounts of DEL could facilitate the ordered array of urethane groups to form more ordered hydrogen bonds. As illustrated in Figure 1, lignin was also capable of coordinating with PU chain segments via hydrogen bonds; therefore, it was beneficial to the ordered aggregation of hard segments. However, when excess DEL was introduced, the steric hindrance effect of lignin would hinder the crystalline chains from forming ordered structures. FT-IR analysis for sample 20% DEL at different temperatures is shown in Figure

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RESULTS AND DISCUSSION Synthesis and Characterization of LPUes. Attributing to the complex molecular structure of lignin, the hydroxyl groups in lignin molecules are sterically hindered, leading to a limited reactivity between lignin and isocyanate groups. Besides, lignin with high molecular weight has a serious aggregation and is difficult to be dispersed in the PU matrix, which leads to poor interfacial compatibility between lignin and the PU matrix.32 To increase the reactivity of lignin, the molecular weight was first reduced by partial depolymerization of lignin in aqueous alkali solution at 155 °C for 8 h. This process was similar to the alkali pulping process.33 As shown in Figure S2a, the weight-average molecular weight of lignin decreased from 4500 g/mol for EL to 1800 g/mol for DEL, along with the polydispersity index reduced from 2.1 to 1.5. Partially depolymerizing lignin under relatively mild condition led to significantly lower molecular weight and narrower molecular weight distribution because OH− promoted the breakage of ester and ether bonds in lignin at high temperature. Meanwhile, as verified by the FT-IR analysis (Figure S2b) and quantitative 31P NMR spectra (Figure S2c), the content of phenolic hydroxyl groups increased from 3.92 mmol/g in EL to 4.36 mmol/g in DEL, which was beneficial to C


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Figure 2. (a) FT-IR spectra of elastomers with different amounts of DEL and (b) FT-IR spectra of sample 20% DEL at various temperatures.

Figure 3. (a) DSC curves of PUes with different amounts of lignin and (b) thermogravimetry analysis (TGA) and differential TG curves of PUes with different amounts of lignin.

2b. With the increase of temperature, the peak intensity at 1720 cm−1 weakened, while the peak at 1681 cm−1 (free urea CO) intensified, which indicated that the increasing temperature could cause fracture in some hydrogen bonds and break the temporarily physical cross-linking network. The gel ratio was measured to evaluate the integrity of the cross-linking network structure of LPUe.36,37 The swelling test results are provided in Figure S3 and Table S2. Although the control sample was synthesized with linear chain extenders, it possessed a gel ratio of 58.0%, indicating the formation of a cross-linking structure under the condition of excess isocyanate in the presence of a catalyst.35 As lignin was introduced into the PU system, the gel ratio increased, indicating the formation of more cross-linking networks in LPUes compared with that in the control sample. Specially, as the DEL content increased, the gel ratio of LPUe first increased and then decreased, with the maximum gel ratio obtained for 20% DEL. This suggested that introduction of appropriate amounts of lignin helped to increase the cross-linking density of PUes thanks to the multifunctional groups in the structure of lignin. However, when excess lignin was incorporated (40% DEL, 60% DEL), the steric hindrance of lignin played a negative effect on the cross-linking density of LPUes, leading to the dissolution of some unreacted lignin from the PU matrix (Figure S3b) in the swelling process and thus the decrease of gel ratio. Thermal Properties of LPUes. The thermal behaviors of LPUes were first analyzed by DSC, as shown in Figure 3a. The characteristic parameters are summarized in Table 2. As lignin could form hydrogen bonds not only with CO and N−H groups in hard segments but also with C−O−C structure in soft segments of PU,12 plus the steric hindrance effect and

Table 2. Characteristic Parameters of Lignin-Based PUes Determined by DSC and TGA

a b

sample

T5%a/°C

Tmaxb/°C

Tc/°C

Tm/°C

ΔHm/(J/g)

control 5% DEL 10% DEL 10% EL 20% DEL 40% DEL 60% DEL

260.3 311.8 303.7 310.6 300.9 284.2 202.5

416.8 414.4 414.5 414.4 415.8 417.3 419.6

−18.6 −23.0 −25.1 −25.1 −27.6 −33.7 −32.1

15.9 13.2 12.7 12.2 12.3 8.4 6.7

34.4 28.8 26.6 24.9 18.6 11.7 6.4

T5% is set as the initial degradation temperature at 5% weight loss. Tmax is the temperature at the maximum degradation rate.

abundant aromatic rings of lignin, the mobility and crystallization ability of LPU chain segments were significantly suppressed by the increasing amount of lignin, leading to a gradually reduced melting temperature Tm and crystallization temperature Tc and smaller melting enthalpy ΔHm. The thermal stability of LPUes is shown in Figure 3b, with the values of T5% and Tmax listed in Table 2. The thermogravimetric process of LPUe could be divided into two major steps: the first step between 200 and 350 °C was related to the breakage of urethane bonds and degradation of lignin, the second step between 350 and 450 °C was due to the degradation of polyether chain segments in polyurethane.31 Increasing the substituting amount of DEL decreased the T5% of LPUe, as lignin started degradation at around 246 °C. The thermal stability of LPUe at high temperature increased with the increasing content of DEL, as verified by the increased Tmax and the residual ash content, which was contributed by the D


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Figure 4. (a) Stress−strain curve of EL and DEL-type lignin-based PUes; (b) stress−strain curves of PUes with different amounts of DEL; (c) mechanical performance of LPUe in this work compared with other research studies and commercial TPU. 1* = Covestro 9370AU, 2* = Covestro 9392A, 3* = Covestro UE-85AEU10FR, 4* = Covestro 372X, 5* = Covestro 1035AU, 6* = Covestro UE-60D10FR, 7* = Covestro U-95AP, 8* = BASF N65AP, 9* = BASF N75A12P, 10* = BASF 598A 53000, 11* = BASF 564D 53000, 12* = BASF 585A 11000.

of DEL was within 40%, the mechanical performance of LPUe was still found at a relatively high level compared with the previously reported LPUes. In addition, compared with commercial TPU elastomer products, the LPUe synthesized in this work exhibited both high tensile strength and high toughness (large elongation at break). While for the commercial TPUs, high strength and large toughness are hard to reconcile. Commercially hard and strong TPU usually exhibits a relatively smaller elongation at break (Figure 4c). The elastomers of 5% DEL−20% DEL exhibited lower modulus than the control sample. This might be resulted by the disruption of lignin to the crystalline regions in the PU matrix,38 which was already verified by the DSC analysis. A more detailed observation disclosed that the tensile curves of samples 5% DEL, 10% DEL, and 20% DEL rose steeply when the strain increased larger than 400%, suggesting obvious strain-induced crystallization.39 The stress whitening phenomenon during stretching was also observed for these samples. However, as the substitution amount of DEL reached 60%, the mechanical performance of the elastomer abruptly decreased, probably due to the serious aggregation of lignin in the PU matrix (Figure S9).13 In general, LPUes exhibited much better elasticity than the control sample, as listed in Table S3. The elastic recovery of LPUe was above 90% when the substitution amount of DEL was no more than 20%. The elastic recovery decreased with the increasing amounts of lignin, which might be due to the fact that the fracture and reconstruction of hydrogen bonds between lignin and the PU matrix dissipated energy during stretching, which restricted the recovery of an elastic network,

aromatic ring structures in lignin molecules. In addition, the sample 10% EL and 10% DEL showed a similar weight loss curve, but the T5% value of 10% DEL was slightly lower than that of 10% EL, which might be due to the fact that lignin with lower molecular weight tended to degrade more easily. Mechanical Performance of LPUes. The effect of the molecular weight of lignin on the mechanical performance of LPUes is shown in Figure 4a. PUes with DEL had a much better comprehensive mechanical performance, especially the doubled tensile strength and toughness than that with EL. This was because DEL had a lower molecular weight and higher content of hydroxyl groups, which was good for its reaction with isocyanate and thus increased the cross-linking density of PUes. On the other hand, DEL with lower molecular weight and higher content of hydroxyl groups exhibited better compatibility with the PU matrix, which was beneficial to the formation of more hydrogen bonds.12 The engineering stress−strain curves of LPUes with different amounts of DEL are shown in Figure 4b, and the characteristic data are listed in Table S3. The PUes of 5% DEL and 10% DEL exhibited the best tensile strength and toughness. Remarkably, the sample of 5% DEL had a tensile strength up to 60.7 MPa and a toughness up to 263.6 MJ/m3 with the elongation at break larger than 1300% and the elastic recovery as high as 96%, which were far better than the control sample without lignin. The mechanical performance of 5% DEL and 10% DEL was among the best LPUe materials, as shown in Figure 4c. As the content of DEL increased, the tensile strength and elongation at break decreased, but Young’s modulus increased significantly and reached the maximum value of 110 MPa for 40% DEL. When the substitution content E


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Figure 5. Tensile hysteresis curves of PUes at the fixed strain of 550% for the blank sample (a) and at the fixed strain of 600% for 5% DEL (b).

and the excess lignin also hindered the movement of polymer chain segments. The hysteresis experiment was performed to explore the energy dissipation of LPUes. The tensile hysteresis curves at fixed strain for the control sample and 5% DEL are representatively shown in Figure 5. The tensile hysteresis curves for other samples are provided in Figure S4 in the Supporting Information. For each sample, evident energy dissipation and relatively high residual strain were observed after the first cyclic stretch. However, after being circularly stretched 10 times, the hysteresis loss and residual strain remained almost unchanged. Remarkably, after heating the LPUe samples at 40 °C for 1 min, the residual strain apparently recovered (Figure 5b), which was benefited from the self-healing of abundant hydrogen bonds after the introduction of lignin. During the heating process, most temporarily constructed hydrogen bonds broke and the elastic network shrinked to its high entropy state. Then, the hydrogen bonds in elastomers recovered to the original state after being cooled, that is why the stress−strain curves and the hysteresis performance recovered to those of the first stretching cycle.40 However, as shown in Figure 5a, the control sample only recovered part of the residual strain and the hysteresis performance did not recover. Besides, as the substitution amount of DEL increased, the residual strain of elastomers increased correspondingly (Figure S4), consistent with the decreased elastic recovery. The hysteresis experiments at different strains were also conducted, and the results are provided in Figure S5. The hysteresis loss increased as the strain increased, indicating that the elastomers continued to dissipate energy during the stretching process. Under the same tensile strain, the hysteresis loss and hysteresis ratio of LPUes increased with the substitution amount of DEL (Figure S5d), confirming that introduction of lignin was helpful to the energy dissipation. This was due to the fact that lignin could form interfacial sacrificial hydrogen bonds with the PU matrix, which could continually break and reform to dissipate energy during stretching and led to the higher hysteresis loss. Reinforcing Mechanism of LPUes. XRD analysis was conducted to study the influence of lignin on the crystallization performance of PUes, and the results are shown in Figure 6. The control sample exhibited a weak crystalline peak at 22.8°, which was attributed to the crystallization of soft segments (PTMEG2000). However, the samples with substitutive DEL all exhibited a wide amorphous peak and the crystalline peak at 22.8° disappeared at 0% strain, indicating that lignin destroyed the ordered structure and weakened the crystallization ability

Figure 6. XRD spectra of LPUes with different amounts of DEL at various strains.

of soft segments. When stretching elastomers to the strain of 500%, a sharp diffraction peak appeared at 20.5° and the peak intensity at 22.8° was enhanced.38,41 Compared with the control sample, the crystalline diffraction peaks of LPUe were stronger and the peak intensity enhanced with the strain (as verified by the sample 10% DEL), confirming that introduction of lignin was helpful to the strain-induced crystallization. As demonstrated by the FT-IR analysis shown in Figure 2a, lignin could construct more hydrogen bonds with the PU matrix, which facilitated the constraint and orientation of chain segments during stretching and led to the significant selfreinforcement via strain-induced crystallization. SAXS analysis at different strains was conducted to further study the deformation mechanism of LPUes. The 2D scattering patterns are shown in Figure S6. The scattering patterns of the control sample and LPUe samples were basically the same before stretching, and the appearance of the 2D scattering ring reflected the microphase separation structure in elastomers.42,43 However, as the content of DEL increased, the scattering ring became bigger, indicating the formation of more nanostructures in elastomers (Figure S6a− d). As the strain increased, the scattering signal changed from homodispersion to orientation to the equator direction, and the scattering ring became rhombic, indicating that crystalline nanophases oriented to the stretching direction.44,45 By applying inverse Fourier transformation to the Lorentzcorrected SAXS curves, the normalized 1D correlation function curves at different strains were acquired and shown in Figure 7.46,47 Calculated from the SAXS 1D correlation function curves, the structural parameters including long spacing (L), crystalline thickness (lc), amorphous thickness (la), and linear F


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Figure 7. SAXS 1D correlation function curves for PUes with different amounts of DEL at 0% strain (a) and at 500% strain (b) and (c) 1D correlation function curves for 5% DEL at 0, 500, and 1000% strains.

more ordered and larger crystals, leading to the selfreinforcement. This was consistent with the XRD results. Overall, the reinforcing mechanism for LPUes is illustrated in Figure 8. First, the polyhydroxyl structure of lignin enabled the formation of a chemical cross-linking network after reaction with isocyanate. Meanwhile, lignin could act as physical cross-linking points via formation of a microphase separation structure in the PU matrix. Furthermore, during the stretching process, the interfacial hydrogen bonds between lignin and PU facilitated the orientation of chain segments and led to strain-induced crystallization and self-reinforcement. The dual cross-linking network and the self-reinforcement during stretching enhanced the strength of PU. The dynamic fracture and reconstruction of hydrogen bonds dissipated energy continually, which improved the toughness of PUes. Compared with other PUes that have similar phenolic urethane bond-based cross-linking networks but without lignin,48,49 the special contributions of lignin in LPUes include the formation of a microphase separation structure in the PU matrix and provide the interfacial dynamic hydrogen bonds between lignin and PU, which could form a strong physical cross-linking network and restrain the PU chain segments to facilitate the strain-induced crystallization, leading to the much stronger tensile strength and larger toughness of LPUe. Reprocessing Properties of LPUes. The urethane bonds could undergo transcarbamoylation reaction at elevated temperature in the presence of the catalyst DBTDL and led to the rearrangement of chemical cross-linking networks (Figure 9a), which endowed PU with excellent thermal reprocessability.50−52 The rearrangement of chemical crosslinking networks in LPUes was confirmed by a rotational rheometer via measuring the variation of shear modulus with time, from which the stress relaxation behaviors of LPUe could be observed, as shown in Figure S7. Relaxation time (τ*) was defined as the time for the modulus of elastomer declining to 1/e of the initial modulus. As the substituting amount of DEL increased, the relaxation time of LPUe increased (Figure S7a)

crystallinity (Φl) are summarized in Table 3, from which the structural evolution of elastomer chains during stretching could Table 3. Structural Parameters of LPUes at Various Strains Calculated from 1D Correlation Function Curves sample control 5% DEL

10% DEL

20% DEL

strain (%)

L (nm)

lc (nm)

la (nm)

Φl

0 500 0 500 1000 0 500 1000 0 500

15.0 15.0 15.0 15.0 15.2 15.0 15.0 15.0 15.0 15.0

4.8 4.5 4.4 4.6 4.7 4.0 4.3 4.8 3.5 4.0

10.2 10.5 10.6 10.4 10.5 11.0 10.7 10.2 11.5 11.0

0.32 0.30 0.29 0.31 0.31 0.27 0.29 0.32 0.23 0.27

be indirectly reflected. As shown in the results, all samples exhibited similar L, while Φl decreased from 32% of the control sample to 23% of sample 20% DEL under 0% strain, confirming the inhibition effect of lignin on the crystallinity of PU. For the control sample, with the increase of strain, lc decreased, while la increased. This was because the crystalline particles were partially destroyed, while the amorphous chain segments were elongated after stretching. For the LPUe samples without stretching, the values of lc and Φl were smaller, while the value of la was larger than those of the control sample, consistent with the XRD and DSC results, further verifying that lignin impeded the ordered crystallization of chain segments because of its steric hindrance effect and hydrogen bonds. However, as the strain increased, the LPUe samples exhibited increased lc and reduced la, in contrast to that of the control sample. This further demonstrated that the hydrogen bonds between lignin and the PU matrix could effectively restrain the amorphous chain segments, which facilitated the chain orientation during stretching and formed G


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Macromolecules

Figure 8. Mechanism for toughening of the PUe by lignin.

Figure 9. Reprocessing properties of PUes with different amounts of DEL: (a) mechanism for the reprocessing of lignin-based PUes; (b) cycle processing of 20% DEL; and (c) stress−strain curves of 20% DEL after the first and second hot pressing. The hot-pressing time was 5 min, and the pressing temperature was 160 °C for both hot processing.

demonstrates the thermal reprocessing ability of LPUe. Molded samples were cut into pieces and hot-repressed in a mold to shape again. The tensile stress−strain curve of 20% DEL after the second hot pressing is shown in Figure 9c in comparison with that after the first hot pressing. The Young modulus, tensile strength, and elongation at break retained 86.1, 72.5, and 87.0%, respectively, after the second hot pressing. The reprocessing curves and the retention ratios of the samples 5% DEL and 10% DEL are provided in Figure S8 and Table S4. LPUe showed some extent decrease in

because of the increased cross-linking density. Figure S7b depicts the representative stress relaxation curves of 20% DEL at different temperatures. With the increase in relaxation temperature, the relaxation rate increased and the relaxation time decreased, indicating that only at elevated temperature the urethane linkages could undergo fast transcarbamoylation reaction.53,54 The stress relaxation behavior that resulted from the ester exchange reactions in urethane linkages enabled the LPUe materials to be thermally reprocessed. Figure 9b visually H


Macromolecules

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mechanical properties after second hot pressing. This might be due to the relatively low efficiency of ester exchange reaction within a limited time.55 The steric hindrance of lignin which impeded the movement of polymer chains could also weaken the rearrangement efficiency of chemical cross-linking networks. However, in general, the LPUe samples synthesized herein could be reprocessed in short time with a relatively high retention ratio of mechanical performance (>70%), especially for the elongation at break and Young’s modulus (86−108%). This was benefited from the relatively high content of hydroxyl groups in lignin because the phenolic urethane bonds could undergo transcarbamoylation reaction more easily,31,56 which facilitated the reconstruction of the cross-linking network. Therefore, it was possible to partially substitute petroleumderived polyether polyols with lignin to synthesize LPUes with thermal reprocessing ability.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.L.). *E-mail: [email protected]. Phone: +86 020-87114722 (X.Q.). ORCID

Weifeng Liu: 0000-0002-0408-1149 Xueqing Qiu: 0000-0001-8765-7061 Author Contributions §

W.L. and C.F. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully thank the National Natural Science Foundation of China (21706082), the Science and Technology Program of Guangzhou (201707020025, 201804010140), and the Natural Science Foundation of Guangdong Province of China (2017B090903003, 2018B030311052) for the financial supports.

CONCLUSIONS

High-performance LPUes were successfully synthesized via partially substituting PTMEG2000 with DEL. The weightaveraged molecular weight of lignin was reduced to 1800 g/ mol via partial depolymerization in alkali solution. The strength and toughness of the PUe were significantly enhanced after the introduction of DEL. The maximum tensile strength of LPUes reached up to 60.7 MPa and the toughness up to 263.6 MJ/m3 with the elastic recovery larger than 95%, which was much better than the control sample without lignin and was among the best LPUe materials. The dual cross-linking network and the interfacial hydrogen bonds between lignin and the PU matrix were demonstrated to facilitate the orientation of chain segments and resulted in obvious strain-induced crystallization, which enhanced the strength and toughness of PUes. Lignin-containing elastomers synthesized in this work could maintain excellent mechanical performance after being hot reprocessed. This work successfully demonstrated that high-performance LPUes with thermal reprocessing ability could be synthesized via partially substituting the petroleumderived polyether polyols with green biomass lignin.

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Article

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b01413. 1D correlation function curve; GPC curves of EL and DEL and FT−IR and 31P NMR spectra of EL and DEL; hydroxyl group contents of lignin calculated from 31P NMR spectra; gel fraction of PUes with different amounts of lignin; swelling tests of PUes with different amounts of lignin; mechanical properties of LPUes; tensile hysteresis curve of PUes with different amounts of DEL at the fixed strain of 600% for 10% DEL and 20% DEL; variational hysteresis curves of PUes with different amounts of DEL; 2D SAXS scattering patterns of LPUes; stress relaxation curves of PUes with different amounts of DEL at 160 °C; comparison of mechanical properties for lignin-based PUes after two times hot pressing; stress−strain curves of PUes after the first and second hot pressing; and TEM images of PUes (PDF) I


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